The present invention relates to optical fiber couplers and, more particularly, to couplers that are adapted for use in optical fiber amplifiers.
In many couplers, it is desired to obtain substantially 100% coupling at a particular wavelength or band of wavelengths. It is also sometimes desirable to use two fibers in a 1.times.2 or 2.times.2 coupler which differ substantially in core index, core diameter, and/or cutoff. For example, published European patent application EP-A-0504479 teaches a fiber amplifier including a gain fiber 10 (see FIG. 1 hereof). A wavelength division multiplexer (WDM) fiber optic coupler 20 couples pump power of wavelength .lambda..sub.P from laser diode 15 and the signal of wavelength .lambda..sub.S from input telecommunication fiber 14 to gain fiber 10. The fiber pigtails extending from coupler 20 are connected to other optical fibers by fusion splices or butt joint connectors, splices 16, 22 being preferred because of their lower reflection and insertion loss. For optimal amplifier operation, the input signal splice loss at splice 16 should be small in order to maximize signal-to-noise (S/N) of the amplifier because in the signal-spontaneous beat noise limit, the electrical S/N of the amplifier depends linearly on the optical coupling efficiency. Also, the loss at splice 22 should be low for both good coupling efficiency (for the same S/N reason stated above) and pump coupling efficiency since amplifier gain is related to the amount of pump power coupled to the gain fiber. A coupling means 24 such as a tapering fiber or a coupler similar to coupler 20 can provide a relatively low loss connection between the gain fiber and an outgoing telecommunication fiber 25.
Gain fibers operate best when the intensities of both the pump and signal beams are high. This can be accomplished by providing the gain fiber with a relatively small mode field diameter (MFD), a characteristic that causes the optical power to be concentrated in a relatively small area along the fiber axis. Such a "high gain" or "high efficiency" fiber can be achieved by employing a relatively large value of .DELTA..sub.1-2 and a relatively small core diameter. The term .DELTA..sub.1-2 is equal to (n.sub.1.sup.2 -n.sub.x.sup.2)/2n.sub.1.sup.2, where n.sub.1 and n.sub.2 are the refractive indices of the fiber core and cladding, respectively. It is also desirable that the cutoff wavelength of the small mode field diameter fiber be below wavelength .lambda..sub.P of the pump source in order to achieve optimal pump signal energy coupling and low noise.
If a conventional WDM coupler were used for coupler 20 and both coupler fibers were commercially available telecommunication fibers, the mode field mismatch between the small MFD high gain fiber and the large MFD fiber would cause high insertion losses at the splice between those fibers. Consider, for example, a telecommunication system employing an erbium doped gain fiber having MFDs of 6.4 .mu.m and 3.7 .mu.m at 1550 nm and 1000 nm, respectively. The gain fiber is capable of amplifying signals at wavelengths between 1530 and 1560 nm; of the various possible pump wavelengths, 980 nm is preferred. If coupler 20 were a conventional WDM coupler, it would typically be formed of matched commercially available telecommunication fibers having MFDs of 10.5 .mu.m and 5.7 .mu.m at 1550 nm and 1000 nm, respectively, for example. Such coupler fibers are chosen to minimize the splice loss to telecommunication fibers. However, a splice between the aforementioned gain and telecommunication fibers would exhibit splice losses of 0.5 dB and 1.7 dB at 1536 nm and 980 nm, respectively. Such splice losses reduce amplifier gain, and they reduce the useable output power of the amplifier.
In accordance with the teachings of EP-A-0504479, the fiber amplifier of FIG. 1 employs a WDM coupler 20 which is formed of two different optical fibers 21 and 13. Fiber 13 is an optical fiber, the MFD of which substantially matches that of telecommunication fiber 14, and fiber 21 is an optical fiber, the MFD of which matches that of gain fiber 10. In fiber amplifier systems wherein the MFD of the gain fiber is sufficiently small to achieve suitable power density, the ratio of the MFD of fiber 13 to the MFD of fiber 21 is typically at least 1.5:1.
The relatively large difference between the MFD's and cutoff wavelengths of coupler fibers 21 and 13 result in a relatively large difference between the propagation constants (.DELTA..beta.) of the fundamental modes propagating in those fibers outside the coupling region. It is noted that a relatively large difference between the cutoff wavelengths of coupler fibers 21 and 13 can also result in a relatively large .DELTA..beta.. The effect of an MFD or cuoff wavelength difference on the propagation constants of the fundamental modes propagating in those fibers inside the coupling region (.DELTA..beta..sub.CR) is not as great. The cores of the coupler fibers can become so small in the coupling region that their effect on propagation becomes very small. When the fiber cladding diameter becomes sufficiently small, the composite of the core and cladding functions as the light guiding portion of the waveguide in the coupling region, and the surrounding low index matrix material functions as the cladding. Power therefore transfers between the adjacent fiber claddings in the coupling region. By controlling the length of the coupling region and the steepness of the transition region between the stretched and unstretched regions of the coupler fibers, if the .DELTA..beta..sub.CR is small enough, the spectral coupling characteristics of the coupler can be made to be such that a high percentage of the signal light propagating in the large MFD fiber is coupled to the low MFD fiber and a low percentage of the pump source light propagating in the small MFD fiber is coupled to the large MFD fiber.
A similar effect occurs in fused biconically tapered couplers wherein the fibers are surrounded by air rather than matrix glass in the coupling region.
In the system of FIG. 1, there is some maximum acceptable value of .DELTA..beta..sub.CR associated with the coupler fiber mismatch that is sufficiently small that coupling is not seriously degraded. A 1540 nm input signal coupled by such a coupler would be greater than some given minimum acceptable value, for example 95%.
However, in order to enhance gain fiber quantum efficiency, gain fibers having a .DELTA..sub.1-2 of at least 2.0% may be required. Assume that the coupler, referred to herein as Coupler A, utilized with that gain fiber comprised the above-identified commercially available telecommunication fiber and a small MFD fiber having a 2.0% .DELTA..sub.1-2 and a cutoff wavelength of approximately 1300 nm. Only 30 to 40% of the 1540 nm input power would couple from the input telecommunication fiber to the small MFD fiber at maximum coupling (see curve 35 of FIG. 3).
It is noted that coupler fiber .DELTA..beta..sub.CR can be caused by differences in fiber characteristics other than .DELTA..sub.1-2, cutoff wavelength and core diameter. A .DELTA..beta..sub.CR sufficiently large to degrade coupling can also occur if (a) the outside diameters of the coupler fibers differ, or (b) the coupler fibers are sufficiently different in composition or geometry that they are deformed differently upon collapse of the tubing during the manufacture of an overclad coupler.
It has been known that varying the refractive index of the fiber cladding in a fused biconically-tapered WDM coupler enhances the wavelength selectivity of the coupler. Also, the coupling from one fiber to the other in a 1.times.2 or 2.times.2 fused coupler increases with wavelength because diffractive mode field expansion increases with wavelength. The rate of change of this coupling with respect to wavelength limits how narrow a wavelength separation can be obtained between maximum and minimum coupling. By adjusting the indices of refraction, the .beta. curves of the fundamental modes of the two fibers in the coupling region can be made to cross at a high angle at some particular wavelength. Because complete coupling can only be obtained when .DELTA..beta..sub.CR &lt;&lt;C, this causes coupling at a wavelength remote from the .beta. "crossover wavelength" to be reduced, thereby sharpening the wavelength dependence of the coupling. The coupling constant C is discussed later (see equation 2). This principle is taught in U.S. Pat. No. 5,129,020 (M. Shigematsu et al.) and in the publication: O. Parriaux et al., J. Optical Commun. 2 (1981) 3, pp. 105-109.
In U.S. Pat. No. 5,011,251 (Miller et al.) the use of cladding index modifiers is discussed in the context of achromatic couplers. The principle taught there is that a .DELTA..beta..sub.CR between otherwise identical fibers can be created by such a means, and that this can be used to improve the achromaticity (wavelength flatness) of such a coupler. The .DELTA..beta..sub.CR discussed in that patent may be wavelength dependent, but no crossing (where .DELTA..beta..sub.CR .fwdarw.0) is discussed.